Ultrasonic measurement of gas pressure and composition for nuclear fuel rods

The objective of this work was to develop an acoustic method giving an easy access to the pressure and the composition of the internal gas mixture in the upper plenum of a standard LWR fuel rod. Work began in 1993 and proof of concept was achieved in 1999. However, the initial problem was not totally solved because of the presence of the upper spring in the LWR fuel rod. This spring mainly induced a large decrease in the acoustic amplitude response and the above solution was not applicable. The method has been revised recently for several aspects: the design of the sensor to optimize the acoustic power injection through the cladding rod and the entire experimental protocol, including the signal processing in the time and frequency spaces. A single λ/2 layer (instead of the standard solution in λ/4) of low impedance material (compared with the transducer and tube wall impedance) was found to be a better matching layer.